Mekanism v7.1.1.122 in combination with Forge v1231 crashes the game on startup. I tested this with nothing but Mekanism installed, so it's NOT an IC2 issue as you stated earlier. Please look into this.

Crash report: http://pastebin.com/KAHG412F

As you can see, the client crashes while loading Mekanism with the error "java.lang.NoClassDefFoundError: mekanism/common/tile/TileEntityChanceMachine".

Same crash with nothing but Forge v1235 and Mekanism v7.1.1.122

http://pastebin.com/kkqawVhB

Strangely enough installing ic2 api fixes this issue too.

Looks like there may be an @Optional issue somewhere - I'll try and fix it as soon as I have the chance

It seems related to this commit: cd10bc8